Core-shell nanoparticles are formed of a core coated with a shell. This coating may be full or partial as shown in the various diagrams a, b, c and d in FIG. 1. In these diagrams, the core is referenced 150 and the shell 160.
For example, FR 2 885 734 (D1) provides a method for producing a Si—C (core-shell) nanocomposite material.
This process includes the following steps: a′) providing a silicon powder obtained by the technique of plasma-enhanced (PECVD) or CO2-laser-assisted chemical vapor deposition, the size of the silicon nanoparticles being less than 100 nm; b′) mixing the silicon powder with a carbon polymer; c′) pyrolysing the mixture.
The Si—C nanocomposite material thus formed is then used to form an anode of a lithium accumulator.
In this method, the performance of step b′) of mixing the silicon powder with the carbon polymer (serving as a precursor of the carbon for subsequent pyrolysis) is crucial.
Indeed, it is known from the prior art that the carbon shell has several functions within the Si—C nanocomposite material. It serves to accommodate variations in volume of the silicon during its lithiation (formation of a lithium/silicon alloy the volume of which is greater than that of the silicon). It also serves to limit the aggregation of nanoparticles during the cycling (charge/discharge) of the electrode. It also provides an electronic percolation network within the electrode. Finally, the carbon mass fraction within the nanocomposite material can be used to reduce the instability of the electrode/electrolyte interface.
In addition, it sometimes happens during step b′) that an undesirable compound forms, such as silicon carbide (SiC).
As a result, when this nanocomposite material is used as an anode for a lithium accumulator, the performance of this anode is not optimized.
Other methods of production of a Si—C nanocomposite material exist, in particular for producing an anode for a lithium accumulator.
Thus, it has been proposed to crush together silicon and carbon (e.g. in the form of graphite) before pyrolysing the previously crushed mixture. This is, for example, the case in the article by Dimov et al., ‘Mixed silicon-graphite composites as anode material for lithium-ion batteries: Influence of preparation conditions on the properties of the material’, Journal of Power Sources, 136 (pp. 108-114), 2044 (D2).
For mixing the silicon and carbon, this type of method requires a lot of energy. However, owing to the required energy level, silicon carbide (SiC) and/or an agglomeration of Si—C nanoparticles are often formed, what is, in all cases, detrimental to the intended application.
The methods provided in items D1 and D2 therefore present limits which have consequences for the electrochemical properties of the finally produced electrode.